It takes YouTube to bring “American Idol” together with Carnegie Hall. And not just because video clips of both are available on the site in profusion. Today, online voting begins to select the members of the YouTube Symphony Orchestra, which will perform at Carnegie Hall on April 15 under Michael Tilson Thomas (pictured) — an ensemble selected entirely from video auditions posted on YouTube, culled by judges in leading symphony orchestras around the world, and chosen in part by votes from YouTube viewers.